Method of injection molding articles of thermoplastic materials

ABSTRACT

A method is proposed of injection moulding hollow formed parts from thermoplastic resins in which first of all an amount of the molten resin sufficient for forming the part is forced into the injection mould. Then, a flowable medium, in particular gas, is forced into the injection mould and/or the thermoplastic resin at a pressure which distributes the resin compound evenly over the surface of the mould cavity, forming a hollow article. Thereafter, the hollow article is cooled in the injection mould whilst kept under the pressure of the medium. Finally, the hollow article is relieved of the pressure of the medium and then the formed part is released and removed from the injection mould. What is essential here is that the pressure of the medium within the injection mould, or on the resin compound distributed over the surface of the mould cavity, is varied at least for a time during its cooling, to be precise by closed-loop and/or open-loop control according to a pressure profile specified for the respective formed part and/or resin material.



